Lymphocyte collection tube

ABSTRACT

This invention relates to an assembly for centrifugally separating lymphocytes and monocytes from the heavier phases in samples of human blood utilizing a liquid density gradient medium. The particular inventive subject matter concerns the use of novel partition means initially separating the liquid density gradient medium from the sample of blood and designs for such partition means.

BACKGROUND OF THE INVENTION

This application is a continuation of application Ser. No. 07/117,396,filed Nov. 2, 1987 now U.S. Pat. No. 4,917,801, which is a continuationof application Ser. No. 06/864,443, filed May 16, 1986, now abandoned,which is a continuation of application Ser. No. 06/678,100, filed Dec.4, 1984, now abandoned, which is a continuation-in-part of applicationSer. No. 06/544,125, filed Oct. 21, 1983, now abandoned.

Considerable research has been conducted in recent years to developimproved means for the separation and collection of lymphocytes fromhuman blood. An impetus for such research has been generated by the needfor histocompatibility determinations in patients requiring organtransplants. A measure of lymphocyte function is critical to adjudge thetype and level of medication necessary for immunosuppression.

One well-known method for isolating and collecting lymphocytes fromanticoagulated human blood drawn via conventional phlebotomy techniquesutilizes buoyant density centrifugation of blood cells. A newtonianfluid, frequently Ficoll-Paque®, a liquid density gradient medium havinga specific gravity of about 1.077 g/cc marketed by Pharmacia FineChemicals AB, Uppsala, Sweden, constitutes the medium. The methodcommonly involves the four general steps:

(a) a predetermined quantity of the Ficoll-Paque® medium is run into thebottom of a test tube;

(b) a sample of whole or diluted blood is carefully pipetted onto themedium;

(c) the test tube is placed in a centrifuge and the blood-mediumcombination centrifuged at about 400-500 G's for about 30-40 minutes tocause the components of the blood having specific gravities greater thanthe medium, viz. >1.077 g/cc, to pass through the liquid; andthereafter,

(d) the lymphocytes, which have a specific gravity less than 1.077 g/cc,are pipetted off the medium.

Several problems or concerns have been found to be inherent in thattechnique. For example:

(1) if, during the pipetting of the blood sample into the separationmedium, lymphocytes are inadvertently diffused below the surface of themedium, the specific gravity of the medium in that area is so reduced asto become inadequate to separate the lymphocytes;

(2) if, during centrifugation, lighter phases in the blood migrate intothe separation medium, they cannot pass upward therethrough because thebuoyant force generated by 400-500 G's is insufficient;

(3) centrifugation forces in excess of about 400-500 G's cannot beemployed with Ficoll-Paque® medium as it is somewhat water soluble andhigher centrifugation forces increase this solubility, thereby leadingto a change in its specific gravity; and

(4) after centrifugation has been completed, the pipetting of thelymphocytes off the surface of the separation medium must be conductedwith substantial care because of the newtonian character of theFicoll-Paque® medium.

Numerous suggestions have been proposed for improving upon thattechnique. Several disclosures of such suggestions are recorded below.

U.S. Pat. No. 3,852,194 describes a process for isolating lighter phasesfrom heavier fractions in human blood utilizing a thixotropic, gel-likematerial having a specific gravity which is intermediate to that of thephases to be separated. Upon centrifuging the gel and blood sampletogether, the gel exhibits sufficient flow to form a barrier between thelighter and heavier phases. That barrier enables the phase restingthereupon to be easily withdrawn therefrom using conventional laboratorytechniques.

The patent postulates the operability of numerous gel-like substances;those substances complying with three general criteria:

(1) a specific gravity intermediate to that of the phases to beseparated;

(2) chemical inertness to the phases of human blood; and

(3) essentially non-flowable when at rest (thixotropic).

U.S. Pat. No. 3,920,549 is asserted to comprise an improvement upon thedisclosure of U.S. Pat. No. 3,852,194. That improvement involved the useof a solid element, termed an "energizer", having a specific gravitygreater than that of the gel-like substance. This energizer, duringcentrifugation, impacts upon the gel, which is normally placed in thebottom of a blood collection tube, thereby expediting the upwardmovement of the gel along the walls of the tube. In this manner theenergizer accelerates the isolation of the blood phases and permits acleaner separation therebetween.

U.S. Pat. No. 4,190,535 is specifically drawn to a procedure forisolating lymphocytes, monocytes, and platelets from anticoagulatedblood. The process contemplates three general steps:

(1) a water-insoluble, thixotropic gel-like substance having a specificgravity between about 1.065-1.077 g/cc and exhibiting chemical inertnessto blood components is deposited into a sample of anticoagulated blood;

(2) the gel-blood combination is centrifuged at a force of at least 1200G's for a sufficient length of time that the gel forms a barrier betweenthe heavier blood cells and the lymphocytes, monocytes, and platelets;and then

(3) lymphocytes, monocytes, and platelets are removed from atop thebarrier.

The patent observes that, because a non-newtonian, water-insolublegel-like material capable of forming a barrier at centrifugation forcesof in excess of 1200 G's is used, a faster and more complete separationwas possible than with Ficoll-Paque® medium. The patent also observesthat the elimination of the liquid density gradient medium avoids thetime-consuming process of layering two liquids without mixing them.

U.S. application Ser. No. 528,401, filed Sept. 1, 1983 in the names ofRichard J. Carroll, Albert A. Luderer, and Anthony R. Zine, Jr., andunder the title of SEPARATION OF LYMPHOCYTES AND MONOCYTES FROM AGEDBLOOD, is directed to improving the quality of the separation oflymphocytes and monocytes from aged samples of anticoagulated humanblood by inhibiting the shift observed in the buoyant density ofgranulocyte white blood cells. The inventive process involves fourgeneral steps:

(1) a sample of anticoagulated blood is mixed with a hypertonic fluidcontaining an organic or inorganic ionic substance of relatively lowmolecular weight and which is chemically compatible with components ofthe blood;

(2) a water-insoluble thixotropic gel-like substance similar to thatdescribed in U.S. Pat. No. 4,190,535 with a specific gravity between1.060-1.075 g/cc is deployed into the blood-hypertonic fluid mixture;

(3) the gel-blood-hypertonic fluid sample is centrifuged at a force ofat least 1200 G's to cause the gel to form a barrier between thelymphocytes and monocytes and the heavier cells of the blood; and then

(4) the lymphocytes and monocytes are withdrawn from atop that barrier.

Whereas each of the above-discussed disclosures does indeed modify andimprove upon various aspects of the well-known Ficoll-Paque® mediumtechnique, none of them is able to equal or improve upon the performanceof the liquid medium with respect to the purity of the separated cellpopulation. Because purity is a critical parameter in cell separation,the above-discussed disclosures cannot be substituted for theFicoll-Paque® medium technique in all applications. Consequently,research has continued in an effort to formulate simpler methods of cellseparation which utilize a liquid medium. More particularly, a processhas been sought which eliminates the time-consuming procedure necessaryto layer blood samples onto the liquid density gradient medium withoutencountering mixing at the interface between the two liquids. Thislayering process generally requires about three minutes/tube to flow theblood sample down the inside wall of the tube at a rate which willpermit layering and avoid turbulence at the interface. Inasmuch as thisprocedure is conducted manually and two tubes are conventionallyprepared per sample, the setup time for readying a group of ten tubesmay require a period of greater than one hour. The time involved in thecentrifuging step is less critical since many tubes can be processed atthe same time. Further simplification of the setup procedure could beaccomplished if the patient's blood sample could be drawn directly intothe centrifuge tube, thereby removing the need for transferring thesample from the collection tube to the centrifuge tube. In manyinstances it is desirable to add a reagent to the blood sample prior tocell separation to anticoagulate the blood, dilute the blood, or modifyphysical and/or chemical characteristics of the blood components.

Therefore, a primary objective of the present invention is to provide aseries of devices which, separately or in combination, will not onlysatisfy the range of needs of research workers and diagnostictechnicians who may merely wish to eliminate the layering problem or tominimize setup time, but also will provide a single product wherein allof the above-described benefits can be enjoyed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a schematic cross section in side elevation of onenon-evacuated tube configuration of the inventive lymphocyte separationtube unit. This tube configuration is employed where a blood sample istransferred from a blood collection tube to the inventive separationtube.

FIG. 2 comprises a schematic cross section in side elevation of oneevacuated tube configuration of the inventive lymphocyte separation tubeunit, which permits blood to be drawn directly from the patient into theinventive separation tube.

SUMMARY OF THE INVENTION

In the most general terms, the present invention comprises an assemblyfor centrifugally separating lymphocytes and monocytes from the heavierphases of a sample of whole blood or a pretreated cell fraction thereofand physically partitioning the separated phases. The inventive assemblyconsists of four basic elements:

(1) a container (customarily a blood collection tube or a centrifugetube) having an open end and a closed end;

(2) a density gradient medium initially positioned adjacent said closedend;

(3) a partition plug initially positioned above the surface of saidmedium which seals said medium therebeneath; and

(4) a free space initially adjacent said partition plug of sufficientvolume to contain said sample and added blood anticoagulant wherenecessary.

A closure means for covering the open end of the container is necessarywhere a sterile product is demanded. Careful practice dictates utilizinga closure means during centrifugation to avoid aerosoling of the bloodwhich may be contaminated with pathogenic materials. For conventionalcentrifuge tubes, a screw top cap is normally sufficient; for evacuatedcollection tube applications, a tight-fitting elastomeric plug isgenerally employed to contain the vacuum during the required storageperiods.

FIG. 1 illustrates a preferred inventive concept while providing aspecific embodiment thereof. Accordingly, to a conventional centrifugetube 1 is added an aliquot of a density gradient medium 2 such asFicoll-Paque®. A stationary partition plug 3 having an aperturetherethrough 4 is then inserted into tube 1 and moved to a positionimmediately above the surface of medium 2. Plug 3 will advantageously befashioned from molded polypropylene having chevron-type sealing meansaround its periphery or molded from an inert elastomeric or plasticmaterial having compression rings around its periphrey. A hydrophobicgel 5 of a selected density and viscosity is injected into the bore 4 ofpartition plug 3, thereby sealing medium 2 beneath. A free space 9 isrequired below the gel 5 which is of a volume approximately equivalentto that of gel 5. Thereafter, an aliquot of a suitable reagent 6, e.g.,a diluent, may be added above partition plug 3. It will be appreciatedthat the use of a blood diluent is not mandatory but appears to promotebetter separation in some cases. A closure 7 is applied to the open endof tube 1 to maintain sterility therein. A free space 8 is left betweenthe closure 7 and reagent 6, that space having a volume somewhat greaterthan that of the volume of the blood sample to be subject to separation.To insure that gel 5 will remain in bore 4 of partition plug 3 so as toretain density gradient medium 2 in place during shipment and storage oftube 1, and where reagent 6 is utilized, to prevent mixing of the twoliquids, a non-newtonian (thixotropic) gel is preferred. Nevertheless,where table 1 is to be used relatively promptly, a newtonian gel may beused.

FIG. 2 illustrates the preferred embodiment of the present inventionwhere the final product is an evacuated blood collection tube. In thisembodiment to a conventional collection tube 10 is added an aliquot of aliquid density gradient medium 11. Partition plug 12 consists solely ofa mass of a hydrophobic (thixotropic) gel which is extruded onto thesurface of liquid medium 11, sealing the medium therebeneath. In likemanner to the embodiment described in FIG. 1, a thixotropic gel ispreferred since it insures long term sealing of medium 11 therebeneathduring shipping and storage. However, a newtonian gel couldself-evidently be inserted into tube 10 on site, and would perform quitesatisfactorily. The gel is sufficiently tacky to adhere to the walls oftube 10. The gel has a specific gravity somewhat greater than that ofliquid medium 11, thereby allowing the gel to move down tube 1 duringcentrifugation and so displace liquid medium 11. An aliquot of asuitable reagent 13 containing a blood anticoagulant, e.g., lithiumheparin, sodium heparin, or EDTA, is added above plug 12. Free space 14between reagent 13 and closure means 15 provides the vacuum and is ofsufficient volume to permit the blood sample to be drawn directly fromthe patent into tube 10. Closure means 15 will conveniently be a stopperfabricated from a special butyl rubber.

As has been observed in this inventive embodiment, the partition plugmoves to the bottom of the tube during centrifugation and, in so doing,displaces the liquid gradient medium. This action makes possible theseparation of cell suspensions which have been previously enrichedthrough prior separation steps. An example of that situation is theseparation of "Buffy Coats". In that protocol whole blood is centrifugedor merely allowed to stand and settle out. The white cell populationforms a "Buffy Coat", i.e., a buff-colored layer, on top of the mass ofred cells. This layer of white cells can be removed, diluted,centrifuged, and partitioned over a gradient density medium to separatethe mononuclear cells. The procedure is conducted as a means forreducing the number of separation tubes required to process anequivalent quantity of cells. The practice permits the separation of ahigh concentration of leukocytes utilizing a small amount of liquidmedium which is very expensive. As can be seen, a mass of red cells todisplace the liquid medium is not necessary in this embodiment of theinventive method, contrary to the first above-described embodiment wherethe partition plug remains stationary in the tube.

Isolation of lymphocytes from blood samples comprehends three generalsteps:

(a) a blood sample is aliquoted or drawn into a tube employingconventional techniques;

(b) the tube is rocked or otherwise agitated to mix the blood with anyrequired reagent; and

(c) the tube is centrifuged in accordance with standard techniques forseparating mononuclear cells (lymphocytes and monocytes) from theheavier phases of blood utilizing a density gradient liquid procedure.

In the first inventive embodiment the gel seal is moved by thecentrifugal force generated, as the tube begins to spin, into the spacealloted below the partition plug. Inasmuch as the liquid medium isincompressible, the plug bore cannot be unsealed if space is notprovided into which the gel can move. In FIG. 1, this space is anartifact of the small size of the bore.

The movement of the gel opens the bore and allows the red cells to passdownward into the liquid medium and, being more dense than the medium,displace the medium upward through the bore to above the partition plug.Since the volume of cells in whole blood is approximately 40%, about 8ml of whole blood will generally displace a typical aliquot of 3 ml ofdensity gradient medium. Where a diluent is employed, care must beexercised in practicing this inventive embodiment to have a sufficientmass of red cells to displace the liquid medium to at least its minimumoperable heighth above the partition plug.

It will be appreciated that the crux of the mechanism operating in thefirst inventive embodiment resides in the aperture of the partition plugwhich confines and controls the interaction of the blood sample with theliquid medium. Hence, this inventive embodiment can be made operablewithout the use of a gel; the gel being a convenient means forfabricating tubes with prepackaged medium and reagent. Furthermore, thisinventive embodiment envisions the use of partition inserts or plugswhich fit standard sizes of centrifuge tubes, thereby enabling users theoption of adjusting the amount of liquid medium desired for specificapplications. This inventive embodiment also contemplates the design ofa separation tube wherein the partition plug is formed as an integralpart of the tube, e.g., as a raised ring projecting inwardly from thewalls of the tube or a constriction in the tube. Each of the aboveoperating modes possesses characteristics which may be of benefit forparticular applications.

It will likewise be appreciated that alternative devices may be devisedto close the bore of the partition plug where it is desired to shipprepackaged liquids. The preeminent requirement therefor is that theunsealing mechanism work unfailingly. For rigid and semi-rigid sealingmeans that work by centrifugal force, very tight control of tolerancesand the elastic properties of the materials is essential. One dependablealternative comprehends the use of a rod to seal the bore; the rodextending upward to the closure of the open end of the tube and havingmeans for grasping, such that when the closure is removed, the rod canbe manually lifted out. In a variation of that alternative, the rod iscapable of being removably attached to the closure such that, when theclosure is taken off the tube, the rod is also removed. The rod is thendetached from the closure prior to the closure being replaced upon thetube for centrifugation. Those alternatives must be so designed,however, that they do not lead to contamination of the sterile tube. Ingeneral, non-manual approaches are favored.

In the second embodiment of the invention the gel pulls away from thewalls of the tube upon centrifugation and moves to the bottom of thetube. This action is sufficiently gradual that the liquid densitygradient medium underlays the blood sample without appreciable mixing ofthe two liquids. The two principal advantages of this inventiveembodiment are its ability to be utilized with "Buffy Coats", and thefact that by out-gassing both the liquid medium and the gel beforeassembly, evacuated blood collection tubes can be pumped down andstoppered on existing evacuation equipment. Thus, such equipmenttypically has the stopper positioned on top of or above the tube on pumpdown, allowing no room for manipulation of partition plugs or boresealing at the pump down station.

Finally, the first embodiment of the invention can also be modified tobe operable with "Buffy Coat" samples. This modification involves theuse of a reagent (6 in FIG. 1) which performs the function of the redcells, viz., it displaces the liquid medium in the bottom of the tube.However, the heavy phase of this reagent must not be such as to applyadditional sealing pressure to the bore which prevents upward movementof the liquid medium. One operable reagent consists of a diluentcontaining a quantity of heavy particles, most desirably glassmicrospheres, having a mass at least equal in volume to that of theliquid medium to be displaced and being inert to the medium and bloodcomponents. Because of the inherent large surface area of the glassparticles, they will be in the blood which can have deleterious effects,such as activating platelets in the blood. A silicone coating applied toglass microspheres operates to preclude reaction between the bloodcomponents and the medium. The inert particles (glass microspheres) mustbe sufficiently small to act as a fluid and not cause a bridging actionabove the partition plug bore. The spherical shape of the microspheresalso avoids any substantial apparent increase in the viscosity of thereagent. Because the coating of the glass particles inhibits chemicalactivation of blood which typically takes place where blood contactsglass, this practice is operable in all applications where a stationarypartition plug is utilized along with gel or rigid plug means to sealthe bore.

Yet another embodiment of the invention comprehends the use of astationary or moveable partition plug fashioned from an integral porousfoam material. A urethane foam has been particularly useful in thatpractice; no attachment of red cells in the foam was observed.

Where a stationary partition plug is employed, the diameter thereof willbe made greater than that of the centrifuge tube such that, wheninserted into the tube, it will be under sufficient compression thatcentrifugation will not dislodge the partition. The porosity of the foamchosen is of such fineness that, when positioned atop a density gradientmedium, the foam will hold the medium in position without movement dueto surface tension. Hence, no mixing of the medium and blood can betolerated when whole or diluted blood samples are poured into thecentrifuge tube. During centrifugation, however, the red cells must passdownward through the foam partition to displace the gradient mediumupward through the partition. Small amounts of medium whichinadvertently pass upward through the partition due to handling,shipping barometric changes, etc., will move back through the foam as aresult of capillary action after the tubes have stood upright for aperiod of time.

Unlike partitions prepared from solid elastomeric materials whichrequire displacement thereof under compressive forces, i.e., they areincompressible, the foams are compressible. Spring constants of the foammaterials are relatively low and tend to be more linear due to bendingof the matrix rather than through compression. Consequently, widevariations in partition diameters are allowable, which circumstancemakes for easy assembly. Moreover, bodies may be die cut from a sheet offoam employing very inexpensive tooling compared with such demanded inworking with plastics and rubber.

The moveable partition plug can be conveniently prepackaged in a drycentrifuge tube. The diameter of the partition is made slightly smallerthan that of the centrifuge tube, permitting it to float upward as thedensity gradient medium is poured into the tube. Two flotationmechanisms are contemplated. The first utilizes a foam having a slightlylighter density than the medium, and the second employs a foam having adensity slightly greater than the medium.

Where the first mechanism is utilized, the porosity of the foam will beof such fineness that some red cells will be entrapped in the poresduring centrifugation. The entrapped red cells will increase theapparent weight of the foam, thereby causing it to move downward as thered cells displace the density medium upward through the partition.

Where the second mechanism is involved, the porosity of the foam isdesigned such that all of the red cells will pass therethrough. Thepartition will float on the density medium for a period of time becauseof the entrapment of small air bubbles as the medium is poured into thetube. During centrifugation those air bubbles are displaced and thepartition moves to the bottom of the tube. Care must be exercised toprevent an excessive quantity of air bubbles which would hazard mixingof the blood sample with the density medium as the air bubbles release.

In both mechanisms the floating partition must be of sufficient lengththat the addition of the blood sample will not spin or tip it. Thediameter of the partition must be such as to permit free movement, butnot so small as to allow mixing of the blood sample and the densitymedium around the perimeter thereof. Most preferably, the blood samplewill be introduced from a pipette at the center of the partition and ata sufficiently slow rate that the blood does not force the partitionrapidly downward into the density medium, resulting in an upsurge ofmedium with consequent mixing with the blood.

Finally, a stationary or a moveable partition plug can be fashioned ofsuch length and volume of porosity as to contain the entire amount ofthe density gradient medium. The use of such a partition would reducethe quantity of medium needed and would better retain the medium duringhandling and shipping. Furthermore, there would be less tendency for"liquid hammer" to dislodge the partition during shipment. The partitionwould also define the foam volume that would be the interface betweenthe density medium and blood.

DESCRIPTION OF PREFERRED EMBODIMENTS

Lymphocyte separation tube units such as are depicted in FIGS. 1 and 2were aseptically prepared by depositing the density gradient medium,Ficoll-Paque®, in the bottom of sterile, siliconized glass orpolypropylene centrifuge tubes followed by placing a silicone-oiled,butyl rubber plug having a bore through the center thereof in contactwith the surface of the medium. Polypropylene partition plugs havingchevron seals on the periphery thereof were also used. Awater-insoluble, thixotropic gel chemically inert to blood constituents,formulated as described in U.S. Pat. No. 4,190,535, supra, from adimethyl polysiloxane and a methylated silica wherein the methylationrenders the gel hydrophobic, and containing fillers to provide specificgravity of 1.085 thereto, was injected into the bore of the plug,thereby sealing the Ficoll-Paque® medium therebeneath. An air bubble wasleft under the partition plug to allow movement of the gel uponcentrifugation. To avoid mixing, the air bubble was designed toapproximate the volume of gel to replace it, so that the medium wouldcontact the blood within the bore of the plug. To simulate reagentadditions, aqueous solutions were placed in contact with the gel forperiods up to several months without changing the properties of the gelsubstantially. 10 ml glass centrifuge tubes and 50 ml plastic centrifugetubes were assembled with partition plugs having holes of various sizesbored therethrough. The resulting assemblies were tested both with thebores open and with the bores sealed with the thixotropic gel. Wholehuman blood samples were pipetted into the tubes without regard forlaminar flow techniques, utilizing fill times of less than 10 seconds.The tubes were immediately introduced into an unrefrigerated table topcentrifuge and centrifuged at about 400 G's for about 30 minutes toachieve equilibrium.

10 ml centrifuge tubes were aseptically prepared by depositingFicoll-Paque® gradient medium in the bottom thereof and placing two mlof the hydrophobic gel described above in contact with and sealing themedium therebeneath. A gel of higher specific gravity was also utilizedwith some tubes.

Examination of the unsealed bore tube showed no mixing occuring betweenthe blood and the liquid medium. Inspection of the tubes with boressealed with gel found that the gel, under centrifugation, had moved downto the bottom of the tube and the medium had moved upward through thebore to assume a position underlying the blood sample. In the tubesutilizing gel alone as the partition, centrifugation caused the gel tomove to the bottom of the tube, thereby displacing the liquid medium. Ineach tube design the Ficoll-Paque® medium was established as a clearcolumn of liquid above the plug. Mononuclear blood cells were seen intheir classic position atop the medium. The plasma fraction of the bloodand the platelets were located at the top of the centrifuge tube.

Subsequently, the plasma fraction was carefully withdrawn (pipetted off)to within a short distance above the Ficoll-Paque® medium such that thelymphocytes and monocytes at the top surface of the medium were notdisturbed. After careful removal of the layer of medium containinglymphocytes and monocytes, those cells were washed and reconstituted inan isotonic buffer solution. Thereafter, the percentage of mononuclearcells contained therein was determined in the conventional mannerthrough hemotoxylin and eosin staining of the fixed cells. Theseseparations were compared against the standard Ficoll-Paque® mediumseparating procedure. The performance results with respect to purity,viability, and yield were essentially identical.

As can be observed from the above, the present invention offerssignificant improvements in ease of use and setup time withoutsacrificing cell purity and recovery.

We claimed:
 1. An assembly for centrifugally separating lymphocytes and monocytes from heavier phases of a sample of whole blood or a pretreated cell fraction thereof which comprises:(a) a container having an open end and a closed bottom end; (b) a liquid density gradient medium initially positioned adjacent said closed bottom end; (c) means for preventing mixing of the liquid density gradient material with a blood sample prior to centrifugation of the container, the preventing means including a partition means initially positioned above the surface of said medium which seals said medium therebeneath, said partition means being movable during centrifugation in the direction of said closed bottom end of said container; and (d) a free space initially adjacent said partition means of sufficient volume to contain said sample and added reagent where desired.
 2. An assembly according to claim 1 wherein said partition means consists of hydrophobic gel having a specific gravity greater than that of said liquid density gradient medium.
 3. An assembly according to claim 1 having a closure means for sealing said open end of said container.
 4. An assembly according to claim 1 wherein said closure means is suitable for vacuum sealing said open end of said container.
 5. An assembly according to claim 1 wherein said closure means is pierceable by a needle for supplying a blood sample to said container which is adapted to draw said sample.
 6. An assembly according to claim 1 wherein a reagent is initially placed on top of said partition means.
 7. An assembly according to claim 6 wherein said reagent consists of a diluent, an anti-coagulant, or a mixture of diluent and anti-coagulant. 